Method and device for determination of a wheel brake parameter

ABSTRACT

A method and a device for determining a characteristic value of a wheel brake are described. The characteristic value here is the brake characteristic value, which representing the relationship between the braking torque or braking force and the brake pressure of a wheel brake operated hydraulically. This brake characteristic value is determined individually for each wheel by calculations based on wheel-specific variables.

BACKGROUND INFORMATION

The present invention relates to a method and a device for determining acharacteristic value of a wheel brake.

In modern control systems for brake systems, brake-specificcharacteristic values are often used. One essential characteristic valueis the proportionality constant between the braking torque exerted onthe wheel brake, i.e., the braking force applied, and the brakingpressure established. This variable is used, for example, with the knownvehicle dynamics control systems. Such a control system is described inthe publication, SAE paper 950759 “VDC—the Vehicle Dynamics ControlSystem of Bosch” by Anton van Zanten, Reiner Erhardt and Georg Pfaff.The setpoint braking torques for a wheel determined by the controlsystem described there are converted into setpoint pressure values byusing a proportionality factor CP, where proportionality factor CPdescribes the brake-specific correlation between the braking torque andthe brake pressure. Furthermore, the brake characteristic values arealso used in the hydraulic model to determine the brake pressures fromthe control signals for the valves, so that the characteristic valueshave an effect on the accuracy of the control. The characteristic valuesplay a role in determination of the wheel performance (e.g., in methodsof calculating slip, etc.) and variables derived therefrom.

The same thing also applies to electrically controlled brake systems inwhich the braking force applied to the wheel, i.e., the braking torque,is adjusted by an electronic regulator as a function of the degree ofoperation of an operating element, in particular a brake pedal, operableby the driver. One example of this is German Patent Application 196 48936 A1, where the brake characteristic values are also analyzed incontrolling the wheel brakes.

It has been found that the brake characteristic values of wheel brakesare not constant but instead vary as a function of temperature, the ageof the brakes, and the condition of the brake disk. The reason for thechange is that these circumstances result in a change in the coefficientof friction between the brake shoe and the brake disk or drum and thusinfluence the relationship between the brake pressure and the brakingforce applied, i.e., the braking torque. If brake characteristic valueswere assumed to be constant, this would result in inaccuracies, whichare unwanted in some applications, in the calculations using thesecharacteristic values or it would result in an incorrect appraisal ofwheel performance.

Therefore, German Patent Application 196 48 936 A1 cited above describesa procedure with the help of which the brake characteristic value and/orvalues are adapted to the changes. On the basis of the equilibrium offorces of the vehicle in the longitudinal direction, a correlation isderived there according to which the prevailing brake characteristicvalue is determined on the basis of the brake pressure and thedeceleration of the vehicle. With the method proposed there, the brakecharacteristic values may be determined only for an axle.

ADVANTAGES OF THE INVENTION

The determination of the brake characteristic value as a function ofwheel slip and additional wheel-specific variables provides anadaptation of the brake parameter, with the help of which continuousadaptation to the respective driving situation and to ambient influencesis possible. This permits a more accurate evaluation of wheelperformance and improves the quality of the estimation of the brakepressure, in addition to improving the regulation and/or control.

It is especially advantageous that the brake parameter is determined foreach wheel only on the basis of wheel-specific variables. Thereforewheel-specific change are determined, resulting in a further improvementin the accuracy of the control.

Additional advantages are derived from the following description and thedependent patent claims.

DRAWING

The present invention is explained in greater detail below on the basisof the embodiments illustrated in the drawing.

FIG. 1 shows a schematic block diagram of a control device forcontrolling the brake system of a motor vehicle.

FIGS. 2 and 3 show flow charts illustrating a preferred embodiment fordetermination and analysis of the brake parameter.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

FIG. 1 shows a control unit, which includes at least one input circuit12, a microcomputer 14, and an output circuit 16. These areinterconnected via a communication system 18 for mutual exchange ofinformation and data. Input lines lead to input circuit 12 so thatsignals which represent the operating variables or from which theoperating variables are derivable are sent to control unit 10. FIG. 1shows as an example input lines 20 through 24, which carry signalsrepresenting wheel speeds VRAD of the wheels of the vehicle, frommeasuring devices 26 through 30. Furthermore, a variable representingthe admission pressure in the brake system is sent from a measuringdevice 32 over an input line 34; the admission pressure is the pressureestablished in the brake system by the driver by operating the brakepedal. Furthermore, input lines 36 through 40 are provided so thatadditional variables, which are of interest in conjunction with thecontrol functions executed by control unit 10, are sent from measuringdevices 42 through 46. Such variables are mentioned in the related artcited in the preamble, for example, and may include the steering angle,yaw rate, transverse acceleration, etc. Output lines 48 through 52 leadfrom output circuit 16 to valve and/or pump arrangements 54 through 58,which are assigned to the individual wheel brakes. As part of thecontrol functions it executes, control unit 10 triggers the valve and/orpump arrangements to influence the brake pressure in the respectivewheel brakes as specified according to the control function.

Examples of the control functions executed by control unit 10 throughprograms implemented in microcomputer 14 are known from the related artcited in the preamble. In the case of braking force and/or brakingtorque calculations based on the brake pressure determined andconversely, the brake characteristic values of the individual wheelbrakes are analyzed when converting setpoint torque values or setpointforce values to brake pressure setpoint values and/or in calculations ofthe estimate of wheel performance. These factors are determined by thedesign of each brake and are composed of the effective contact areas ofthe brake shoes on the brake disks, the distance from the center of eachbrake shoe to the brake disk, the distance from the center of each brakeshoe to the center of the wheel, and the coefficient of friction betweeneach brake shoe and the brake disk. These variables are not usuallyavailable and/or they are subject to changes. If typical values for thebrake characteristic value are assumed as constant, the accuracy ofcalculations performed on the basis of this brake characteristic valueis not adequate for some applications. It has been found that duringphases in which the pressure in the wheel brake is known, it is possibleto estimate the coefficient of friction on the brake and thus toestimate the brake parameter. The pressure in the wheel brake is knownwhen it measured directly or when the driver brakes without regulatingintervention. In this case, the wheel brake pressure is equal to themeasured admission pressure, because the pressure values input by thedriver into the hydraulic brake system are transmitted to the wheelbrakes without any influence. If this is the case, then the brakecharacteristic value may be determined individually for each wheel as afunction of wheel-specific variables. The following formula has provensuitable:CP=(λ*FN*RRAD*CLAM)/PRAD+MMOT/PRADwhere λ denotes the wheel slip, FN is the tire contact force, which maybe measured or estimated, for example, PRAD is the dynamic wheel radius,which may be assumed to be constant, CLAM is the proportionality factorbetween the slip and the coefficient of friction, which may also beassumed to be constant, PRAD is the wheel brake pressure, which in thepresent operating state corresponds to the measured admission pressure,and MMOT is the driving torque of the engine, which is transferred tothe wheel in question. For a non-driven wheel, the additive term istherefore eliminated without a replacement.

If a condition prevails under which the brake characteristic value maybe estimated while braking, the individual brake characteristic valuesare calculated according to the formula given above. To counteractinterference with the signals used, the calculated value is then alsolow-pass filtered using a very large time constant. This eliminatesextreme values in the adaptation.

If the operating state as described above is not available foradaptation of the brake characteristic value, the previously calculatedvalue is retained.

The above formula is derived on the basis of the equilibrium of torqueson the wheel. In another embodiment, an equation derived from theequilibrium of forces is used. These equations differ only by abrake-specific constant.

In the preferred embodiment, the procedure described above isimplemented as a program in microcomputer 14 of control unit 10. Such aprogram is diagramed in FIG. 2 on the basis of a flow chart.

After starting the program part, the required variables are input foreach wheel in a first step 100, i.e., wheel slip λRADi, admissionpressure PVOR, tire contact force Fni and, if available, brake pressurePRADi (measured, or calculated by using the hydraulic model). Then instep 102, a check is performed to determine whether the admissionpressure is 0, i.e., the driver is not braking and a regulator whichmight be present is not active, i.e., no braking intervention is beingimplemented. If this is the case, then according to step 104, previousbrake characteristic value CP(n−1) is retained. Then in step 106, thebrake characteristic value thus determined is filtered through alow-pass filter and output in step 108. This program is repeated in thenext interval. If step 102 has shown that a braking operation isunderway, then in step 110, brake characteristic value CP(n) iscalculated on the basis of the formula given above. This is followed bystep 106.

In the diagram according to FIG. 2, the wheel brake pressure is known.If this is not the case, then step 110 is initiated only when theadmission pressure is greater than a certain limiting value and noregulator is active. In this case, the wheel brake pressure is thenequal to the measured admission pressure.

FIG. 3 shows a flow chart illustrating one possibility for analyzing thebrake characteristic value as a computer program. According to this,after the program starts in first step 200, the brake setpoint valuesdetermined are input for individual wheel brakes MSOLLXY. In the nextstep 202, a corresponding brake pressure setpoint PSOLLXY is determinedfrom the corresponding setpoint torque variable, taking into accountadapted brake characteristic values CPXY. Then in step 204, by using thehydraulic model known from the related art cited in the preamble,triggering times TXY for the valve arrangements are determined for eachindividual wheel brake on the basis of the setpoint pressure thusdetermined and the estimated or measured actual brake pressure.

By using the corresponding variables, the procedure described above isalso used with pneumatic wheel brakes or brakes using an electric motorfor application of braking force. Instead of the brake pressure, anelectric current quantity is then used, for example, because thisrepresents the control variable triggering the application of brakingforce.

1. A method of determining a characteristic value of a wheel brake, thecharacteristic value representing a relationship between one of abraking torque and a braking force and a control variable of the wheelbrake which triggers an application of the one of the braking torque andbraking force by the wheel brake, the method comprising: determining thecharacteristic value as a function of operating variables, and for eachwheel individually as a function of a wheel-specific variable, whereinthe characteristic value is determined only when no regulatorintervention measure is implemented and a driver is braking.
 2. Themethod of claim 1, wherein the control variable is one of a brakepressure and a triggering variable of the wheel brake when actuated byan electric motor.
 3. The method of claim 1, further comprising:filtering the characteristic value.
 4. The method of claim 1, furthercomprising: using the characteristic value to one of determine asetpoint brake pressure from a setpoint braking torque, determine awheel brake pressure from one of a measured braking force and a measuredbraking torque, and estimate wheel performance.
 5. The method of claim1, wherein the characteristic values are adapted based on the operatingvariables for a respective driving situation.
 6. A method of determininga characteristic value of a wheel brake, the characteristic valuerepresenting a relationship between one of a braking torque and abraking force and a control variable of the wheel brake which triggersan application of the one of the braking torque and braking force by thewheel brake, the method comprising: determining the characteristic valueas a function of operating variables, and for each wheel individually asa function of a wheel-specific variable, wherein the wheel-specificvariables include at least one of wheel slip, tire contact force, wheelbrake pressure, and wheel radius, and wherein the characteristic valueis determined only when no regulator intervention measure is implementedand a driver is braking.
 7. The method of claim 6, wherein thecharacteristic values are adapted based on the operating variables for arespective driving situation.
 8. A method of determining acharacteristic value of a wheel brake, the characteristic valuerepresenting a relationship between one of a braking torque and abraking force and a control variable of the wheel brake which triggersan application of the one of the braking torque and braking force by thewheel brake, the method comprising: determining the characteristic valueas a function of operating variables, and for each wheel individually asa function of a wheel-specific variable; and analyzing an engine torqueacting on a driven wheel, wherein the characteristic value is determinedonly when no regulator intervention measure is implemented and a driveris braking.
 9. The method of claim 8, wherein the characteristic valuesare adapted based on the operating variables for a respective drivingsituation.
 10. A method of determining a characteristic value of a wheelbrake, the characteristic value representing a relationship between oneof a braking torque and a braking force and a control variable of thewheel brake which triggers an application of the one of the brakingtorque and braking force by the wheel brake, the method comprising:determining the characteristic value as a function of operatingvariables, and for each wheel individually as a function of awheel-specific variable, wherein a change in the characteristic valueoccurs only in a predetermined operating state, so that outside thepredetermined operating state, a determined characteristic value isretained, and wherein the characteristic value is determined only whenno regulator intervention measure is implemented and a driver isbraking.
 11. The method of claim 10, wherein the characteristic valuesare adapted based on the operating variables for a respective drivingsituation.
 12. A method of determining a characteristic value of a wheelbrake, the characteristic value representing a relationship between oneof a braking torque and a braking force and a control variable of thewheel brake which triggers an application of the one of the brakingtorque and braking force by the wheel brake, the method comprising:determining the characteristic value as a function of operatingvariables, and for each wheel individually as a function of awheel-specific variable, wherein the characteristic value of CP isdefined by the following equation: CP=(ÿ*FN*RRAD*CLAM)/PRAD+MMOT/PRAD.13. The method of claim 12, wherein the characteristic values areadapted based on the operating variables for a respective drivingsituation.
 14. A device for determining a characteristic value of awheel brake, the device comprising: a control arrangement to implementat least one control function by analyzing the characteristic value of awheel brake which represents a relationship between one of a brakingtorque and a braking force and a control variable which triggers anapplication of the braking force, the control arrangement beingconfigured to determine the characteristic value individually for eachwheel based on a wheel-specific variable, wherein the controlarrangement is configured to determine the characteristic value onlywhen no regulator intervention measure is implemented and a driver isbraking.
 15. The device of claim 14, wherein the characteristic valuesare adapted based on the operating variables for a respective drivingsituation.